Wide band phase lock spectral purity measuring apparatus

ABSTRACT

For phase locked measurements of spectral purity of radar transmitters, a comb generator provides harmonics of a crystal oscillator output signal which are applied through a phase shifter and a variable filter to furnish a reference frequency. The transmitter output signal is combined with the reference frequency in a mixer whose IF output is applied to a phase detector along with the output signal of a voltage-controlled oscillator. After rough adjustment of the oscillator, the phase detector output is fed back through a DC amplifier and low pass filter to the control input of the oscillator to maintain quadrature phase lock allowing FM sideband detection. The phase detector output may be passed to a wave analyzer and recording means or passed through a set of tuned bandpass filters and respective threshold detectors arranged to drive indicator means.

fisq "WIDEBA ND PHASE LOCK SPECTRAL $115,934 11/1965 PURITY MEASURING APPARATUS r 1 in] I mom Eldon L on Buck City John K Smith Primary Exgngingr-Edwarfi Kubaszewitz nve s A I l- Philadelphia; John A. Kaeergls, Bucks Home}, ciasclazan em) 3186" Sallen .Q ..324m

City, all ofPa [s71 I ABSTRACT ag i 5555811565 United Sum For phase locked measurements of spectral purity of radar -r 1 "Presented y them?! transmitters,acomb generator pro ides harmonics of a crystal .1. A 1' 1 oscillator output signal which are applied through a phase 17- i lab, 8 970 r so v shifter and a variable filter to furnish a reference frequency. f. {21] Appl. N .1 4 V The transmitter output signal is combined with the reference f 7 1 I l frequencyin arnixer whose IF output is applied toaphase de- Q? l tector along with the output signal of a voltage-cantrolled 1 r 534; 2 oscillator. After rough adjustment of the oscillator, the phase {58] 7 new I 324/77 79 32 83 detector output is fed back through a'DC amplifier and low pass filter to the control input of the oscillator to mam tain i l a A 1- quadrature phase lock allowxng FM sadeband detection. The Ref CM phase detector output may be passedto a wave analyzer and j t UNITED STATES PATENTS recording means or passed through a set of tuned bandpass fil- 5 n ters and respective threshold detectors arranged to drive int 3,154,741 l0[l954 Attwood ..324l83 UX diam, mama 3,059,187 10/!962 .lafie .324/79 UX i 2,888,643 Q5/1959 Summerhayes, Jr ..324l82 l0 Claims,2Drawing Figure XMITTER REFERENCE saunas V l 14 I6 r :5 ts I 11 ceYs'rAL coma Pans: VARKABLE use. A GENERATOR SHIFTER FILTER {:2 2a 80x58 V SIDE-BAND DETECTION a u/v/T t as. 2 21 R wave PHASE I ORDER ANALYZER ocrscron FEEDBACK 23 com moL ma UN! 7 LOW PASS VOLTIGE 1 H CONTROLLED I L osc.

SCOPE WIDE B ND PHASE LOCK SPECTRAL Pulu'rv MEASURING APPARATUS STATEMENTOF GOVERNMENT INTEREST The invention described herein may-be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION The invention pertains generally to the field of frequency measurement devices and in particular to test equipment for analysis of spurious FM sidebands.

In a pulsed or CW doppler radar system, the frequency difference between the transmitted signal and the returned echo is related to the transmitting frequency and the relative speed of the reflective target object. For relatively slow moving targets, the echo will be returned at a frequency very close to the transmitting frequency. Doppler radar transmitters, therefore, must have a good spectral purity, ideally emitting a single,

sharply defined frequency. lf FM sideband noise close to the nominal transmitting frequency is present, the spurious spectrd lines may appear in the echo signal as false moving targets. Moreover, a dense spurious spectrum may mask real targets and degrad the reliability of detection.

SUMMARY OF THE INVENTION 'tection. A further object of the invention is to detect FM noise with greater sensitivity close to the carrier frequency. Still another object of the invention is to provide a technique of wave analysis enabling measurements in narrower bandwidths with greater selectivity than formerly possible with common spectrum analyzers. Another object of the invention is to use a selectable pure frequency reference and to phase lock with the carrier signal regardless of drift.

These and other objects of the invention are achieved by using a comb generator to provide harmonics of a crystal oscillator output which are applied through a phase shifter and a filter to a mixer receiving the attenuated output signal of the transmitter to be tested. The mixer lF output signal is applied to a phase detector along with the output of a voltage-controlled oscillator. The phase detector output is fed back after rough adjustment of the oscillator via a DC amplifier and lowpass filter to the control input of the oscillator. The phase detector output is passed to a wave analyzer and recording apparatus. in place of the wave analyzer, an automatic selectable bandwidth monitoring system may be provided by a plurality of bandpass filters tuned to successive bandwidths containing doppler frequencies of interest. The filter outputs may be pased to respective threshold level devices arranged to drive indicator means.

BRIEF DESCRlPTlON OF THE DRAWINGS FIG. 1 is a block diagram of spectral purity measuring apparatus according to the invention; and

Fit]. 2 is a block diagram of another embodiment of the sideband detection unit of FIG. 1.

' DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, the output of a transmitter whose source 13 purity is to be measured is passed via an attenuator 11 to one input of a mixer 12. The other input to mixer 12 is from a reference 13 comprising a crystal oscillator 14 whose single frequency output of known spectral purity is fed to a comb generator. 16 producing a frequency spectrum consisting of multiples of the crystal oscillator frequency. Generator 16 may comprise a coaxial, microwave, step recovery diode module whose multiple output signals are at a frequency ap proximately equal to the input frequency. The output of comb generator 16 is passed to a phase shifter 17 to maximize the power at the output of mixer 12. At microwave frequencies filter I8 is passed to theother input of mixer 12 via an isolator l9. Mixer 12 then combines the two input signals to producean lF signal. A Shottky barrier diode is preferably used for mixer 12 instead of a point contact diode since the latter type exhibits l/f.'noise creating severe deterioration for low lF output frequencies.

The IF output of mixer 12 is passed to a phase detector 21, a suitable example of which is a Shottky barrier diode phase de tector. Phase detector 21 operates like a mixer beating the input frequencies together, but its output contains an essentially DC component related to the phase difference of the two input signals. This DC component changes from a slowly varying signal to a steady or nulled signal when the input signals are in quadrature, that is, out of phase with each other. Those skilled in the art will recognize that the output of phase detector 21 will contain FM sidebands with no AM noise only when the input signals are maintained in quadrature. The other input of phase detector 21 is connected to the output of a voltage-controlled oscillator 22 preferably capable of manual as well as closed loop control. I A feedback loop from the output of phase detector 21 to the control input of oscillator 22 comprises a feedback control unit 23 having a DC amplifier 24 whose output is passed through a low-pas filter 25 to provide a control voltage input to oscillator 22. Amplifier 2d may be a conventional amplifier designed to pass DC as well as AC signals. A suitable amplifier circuit would, for example, have a direct-coupled input stage, variable gain and some means of varying the output level. Low-pass filter 25 may be of the conventional RC type grounding out AC signals above, for example, 30 Hz. so that the control input of oscillator 22 will follow on y relatively slow changes in the DC level representing phase shifts between the inputs to phase detector 21.

The unfiltered output of amplifier 24 is connected to an oscilloscope 27' to monitor the output of phase detector 21. When the inputs to phase detector 21 are not in quadrature, the output of amplifier 24 will appear as a low frequency sine wave. When the inpuL are in quadrature a straight line is approximated indicating the null condition.

The output of phase detector 21 is passed via junction 26 to a sideband detection unit 28 including a wave analyzer 29 which sweeps the output bandwidth of phase detector 21 with a narrow band-pass filter having a window of, for example, 10 Hz. bandwidth. The rate at which the window is moved depends on the bandwidth of the window, smaller bandwidths requiring slower movement. The output of wave analyzer 29 is pased to a recording device 31 which indicates the power level of the FM sidebands. The slower the sweep, the more detailed the account of FM sideband power. The recorded output may be calibrated in db. relative to the known power level of the carrier signal. if desired, an additional DC null meter may be connected to the phase detector output at junction 26. Like oscilloscope 27 its nulled reading indicates quadrature. Referring now to FIG. 2, the readout portion of an operational test set is depicted for repeated use with transmitters of specified frequencies. To enable untrained personnel to check the spectral purity of an in-use doppler radar transmitter, the

wave analyzer 29 and recorder 31 of sideband detection unit I 28 are replaced by aplurality of band-pass filters 33 fed in parallel by the output of phase detector 21. Filteis 33 can be set to pass chown frequencies that are of interest for evaluation because they lie in the doppler range, or a completely 3 tunable filter channel can be used to investigate the broadband spectrum. The outputs of filters 33 are passedto respective threshold detectors 34 which produce outputsonly when specific signal thresholds have been exceeded. The outputs of threshold detectors 34 are next passed via respective amplifiers 35 to an indicator 36 which may comprise a plurality of GO-NO GO lights, meters or an oscilloscope having selectable inputs corresponding to each bandpass filter 33. The passbands for filters 33 may be chosen in accordance with the doppler frequency range of interest. For example,filters 33 may cover a continuous, successive range of frequencies. One of the filters 33 may be a selectable filter having a constant passband width which may be swept over the frequency range of interest in operation, assuming a CW transmitter operating at the X-band frequency of 9.68 gl-lz., the proper reference source frequency should be chosen so that the mixer 12 output frequency is in the range available from oscillator 22. if crystal oscillator 34 has a 100 MHZ output the appropriate microwave signal is selected by adjusting variable filter 18 to pass the 97th harmonic signal produced by comb generator 16. The output of the reference source will thus be 9.7 gHz. which when mixed with the attenuated transmitter 10 frequency of 9.68 glla will produce an IF signal output from mixer 12 of 20 MHz assumed here to be well within the bandwidth of Oscillator 22.

Once the appropriate IF replica of the transmitter signal has been obtained, oscillator 22 should be manually adjusted so that its output is approximately at the same frequency as the IF. During this initial rough tuning, the feedback loop is open,

' that is, the frequency is not closed loop controlled. Oscilloscope 27, displaying the amplified output of phase detector 7 21, can be used to identify the null condition when oscillator 22 is slowed to a position close to the carrier frequency in the ll? signal. This preliminary procedure is necessary due to the relatively narrow bandwidth response of the control input to oscillator 22 because of low-pass filter 25. Slow DC changes will be followed, but relatively high frequencies in the phase detector 21 output will be ignored by oscillator 22. Thus, phase lock is not possible unles oscillator 22 is first brought into rough agreement with the PF inprlr Qggg apggggimgt: nulling is observed, the feedback loop may be closed allowing servocontrol of the output of oscillator 22.

By adjusting the output level of amplifier 24, phase lock (an be verified. lfthe input frequencies to phase detector 21 are not close enough, varying the amplified feedback signal will have little effect. If, however, the signals are approximately at the same frequency, initial nulling can be obtained with the feedback loop closed by adjusting the output level of amplifier 24. With the inputs to phase detector 21 in quadrature, the de tector output willcontain FM sideband signals which can be measured by wave analyzer 28.

The operational test set functions in a similar manner except for the readout portion of FIG. 2. lfthe test set is to be used repeatedly to test the same nominal transmitter frequency, oscillator 22 may be preadjusted so that it will be in the range for positive phase lock, thus eliminating initial, openloop adjustments. Band-pass filters 33 may be set to known doppler ranges. lf GO-NO 60 lights are used, a NO 60 light for a specific filtered band would indicate that the transmitter itself was contributing unacceptable, false doppler spectra which might disqualify the transmitter for use in the doppler radar.

Numerous advantages are achieved by the present invention. The apparatus of FIG. 1 provides means for relating FM sideband power to carrier power for gauging the spectral purity of signal sources. By using the frequency-sweeping wave analyzer 29 and recorder 31, a catalog of spectral content can be generated. The phase lock feedback loop makes continuous manual frequency adjustment due to drift unnecessary and enables operation by untrained personnel. Reference source 13 provides a wide-band capability. By changing the bandwidth of filter l7, frequencies .from the VHF band through X band (100MHz to 12.4 gHz. using l0!) MHz oscillator i4) are available for use with transmitters having corresponding frequencies. The system is thus by no means limited to the microwave region, although that region is of high interest in doppler applications. One central advantage of the invention is that the reference source need not be precisely at the same frequency as the equipment to be tested since an IF frequency is generated. Quadrature is accomplished at level, making the reference the IF level instead of the RF source far more versatile. l

Another advantage lies 'in the relatively uncomplicated nature of the equipment. The test apparatus is comparatively inexpensive, lightweight and inherently stable in the aircraft en- 7 vironment. Moreover, t spectral purity measuring apparatus of the invention exhibits improved sensitivity and selectivity over prior art devices.

lt will be understood that various changes in the details, been nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as exprcsed in the appended claims.

What is claimed is: r 1. Spectral purity measuring apparatus comprising:

means for providing a predetermined fixed frequency; 5 means adapted to receive said fixed transmitter output for producing an i? replica output of said transmitter output; variable oscillator means for producing an output frequency whose phase is variable by means of a control signal in- P phase detector means having a first input connected to receive the IF replica output and a second input connected to receive the variable oscillator means output for producing an output indicative of phase quadrature of said first and second phase detector means inputs; :eedback means connected to receive phase detector means output for producing said control signal input to said variable oscillator means; and

means connected to receive the phase detector means put for indicating bandwidths.

2. Spectral purity apparatus comprising:

means adapted to receive an RF transmitter output for producing an IF replica output thereof including a fixed frequency oscillator, means connected to receive the output of said fixed frequency oscillator for generating harmonies thereof, filter means connected to receive the output of said harmonic generating means for producing an output containing selected ones of said harmonics, mixer means having a first input adapted to operativcly r iv: the RF transmitter output and a second input operatively connected to receive the filter means output for combining the signals at said first'and second mixer means inputs to produce said IF replica output;

variable oscillator means for producing an output frequency whose phase is variable by means of a control signal itr outthe signal level thereof in a plurality of phase detector means having a first input connected to i receive the IF replica output and a second input connected to receive the variable oscillator means output for producing an output indicative of phase quadrature of said first and second phase detector means inputs;

feedback means connected to receive the phase detector means output for producing said control signal input b said variable oscillator means; and

means connected to receive said phase detector means 0utput for indicating the signal level thereof in'a plurality of bandwidths.

3. Measuring apparatus according to claim 2 wherein:

said means for indicating the signal level comprises a wave analyzer connected to receive said phase detector means outputv and producing an output representing the lcvelof frequency and an RF I said phase detector means output as a function of signal input, said phase detector means second input frequency and recording means for indicating said wave being connected to receive the output of said voltageanalyzer output. controlled oscillator;

4. Measuring apparatus according to claim 2 wherein: amplifying means connected to receive said phase detector said signal level indicating means comprises a plurality of 5 means output; band-pass filters each connected to receive said phase defilter means connected to receive'the output of said amplifytector means output for producing respective outputs ining means for providing said control signal input to said dicating the signal level within each respective filtered voltage-controlled oscillator; band, a plurality of threshold detectors connected respeca plurality of band-pass filters connected respectively to tiveiy to receive the outputs of said plurality of bandpass l receive said phase detector means output; and

filters for providing respective output signals when the aplurality of threshold detector means connected to receive outputs of said band-pass filters exceed respective the respective outputs of said band-pass filters for providthreshold levels and display mean operatively connected ing respective outputs indicating whether the levels to receive the outputs of said plurality of threshold detecwithin the filtered bands exceed respective thresholds, tors for indicating the respective conditions of the l the utput f said plurality of threshold detector means threshold detector outpum. being adapted to be connected to display means for in- 5. Measuring apparatus according to claim 2 wherein: dicating the conditions of the respective-threshold detecsaid feedback means includes amplifying means connected tor means outputs.

to receive said phase detector means output and low-pas 5*. Apparatus according to claim 8 wherein: filter means connected to receive the output of said am- 2 i mixer means is a Shottlry barrier diode. plifying means for producing said control signal input to f for mcasfmng Spectral content said frame oscmamr meal ternal signal having a nomina carrier frequency, composing:

, 6. Measuring apparatus according to claim 5 further coznfixed frequency oscmamr means;

prising; I comb generator means connected to receive the output of means operatively connected to receive said phase detector I the F fixer frequency oscnamr for pmv'dmg means output for indicating whether said first and second nfcmcs thereof; inputs to said phase detector means am in phase quadm variable filter means operatrvely connected to receive the mm output of said comb generator means for selecting 7. Measuring apparatus according to claim 6 wherein: Pmdemmmed ones Ofsald hammmcs;

said means for indicating phase quadrature comprises null F having first input Pperafivcly-connected to detection means receive the output of said variable filter means and a 8. Apparatus for measuring the spectral content of a transsecmld adapted m f Said enema] 98 miner wmuncomprgsing; combining the signals at said first and second inputs to I a fixed frequency oscillator; produce an [F output q fl comb generator means connected to-receive the output of Phase detector mam having first and second inputs for said fixed frc uenc oscillator for n cduc' harmonics producing indigmve of the phase. thereof; q y mg between the signals at said first and second inputs, said variable filter means operatively receiving the output otsaid bemg Fonnected m wave the output of comb generator means for selecting predetermined ones mixer Sad Phase h means bclng ofsaid harmonics; 40' adapted to be connected to display means for indicating mixer means having a first input adapted to operafively the level of said phase detector means output in a pluralireceive the transmitter output and a second input cmra- Dumas band"".:d=h5; a tively connected to receive the output of said variable vollage'cflmmned Osclllfltot ans f r producmganoutput filter means for combining the signals at said first and f 'l phase vanable by means ofa 69mm! second inputs to produce an lF replica of said transmitter '3 input phase, detactormeans f Input output; being connected to receive the output of said voltagephase detector means having first and second inputs for f f oscmamr means;

producing an amp: indicative of the phase difference amplifying means connected to receive said phase detector between the signals at said first and second phase dctecmm outputand t mt means inputs. said first input being connected w filter means connected to receive the output ofsatd ampltfyreceive the mixer means output; mg means for providing satd control signal input to said voltage controlled oscillator means for producing an output voltage manned osclnamr me frequency whose phase is variable by means of a control 

1. Spectral purity measuring apparatus comprising: means for providing a predetermined fixed frequency; means adapted to receive said fixed frequency and an RF transmitter output for producing an IF replica output of said transmitter output; variable oscillator means for producing an output frequency whose phase is variable by means of a control signal input; phase detector means having a first input connected to receive the IF replica output and a second input connected to receive the variable oscillator means output for producing an output indicative of phase quadrature of said first and second phase detector means inputs; feedback means connected to receive phase detector means output for producing said control signal input to said variable oscillaTor means; and means connected to receive said phase detector means output for indicating the signal level thereof in a plurality of bandwidths.
 2. Spectral purity apparatus comprising: means adapted to receive an RF transmitter output for producing an IF replica output thereof including a fixed frequency oscillator, means connected to receive the output of said fixed frequency oscillator for generating harmonics thereof, filter means connected to receive the output of said harmonic generating means for producing an output containing selected ones of said harmonics, mixer means having a first input adapted to operatively receive the RF transmitter output and a second input operatively connected to receive the filter means output for combining the signals at said first and second mixer means inputs to produce said IF replica output; variable oscillator means for producing an output frequency whose phase is variable by means of a control signal input; phase detector means having a first input connected to receive the IF replica output and a second input connected to receive the variable oscillator means output for producing an output indicative of phase quadrature of said first and second phase detector means inputs; feedback means connected to receive the phase detector means output for producing said control signal input to said variable oscillator means; and means connected to receive said phase detector means output for indicating the signal level thereof in a plurality of bandwidths.
 3. Measuring apparatus according to claim 2 wherein: said means for indicating the signal level comprises a wave analyzer connected to receive said phase detector means output and producing an output representing the level of said phase detector means output as a function of frequency and recording means for indicating said wave analyzer output.
 4. Measuring apparatus according to claim 2 wherein: said signal level indicating means comprises a plurality of band-pass filters each connected to receive said phase detector means output for producing respective outputs indicating the signal level within each respective filtered band, a plurality of threshold detectors connected respectively to receive the outputs of said plurality of band-pass filters for providing respective output signals when the outputs of said band-pass filters exceed respective threshold levels and display mean operatively connected to receive the outputs of said plurality of threshold detectors for indicating the respective conditions of the threshold detector outputs.
 5. Measuring apparatus according to claim 2 wherein: said feedback means includes amplifying means connected to receive said phase detector means output and low-pass filter means connected to receive the output of said amplifying means for producing said control signal input to said variable oscillator means.
 6. Measuring apparatus according to claim 5 further comprising: means operatively connected to receive said phase detector means output for indicating whether said first and second inputs to said phase detector means are in phase quadrature.
 7. Measuring apparatus according to claim 6 wherein: said means for indicating phase quadrature comprises null detection means.
 8. Apparatus for measuring the spectral content of a transmitter output, comprising: a fixed frequency oscillator; comb generator means connected to receive the output of said fixed frequency oscillator for producing harmonics thereof; variable filter means operatively receiving the output of said comb generator means for selecting predetermined ones of said harmonics; mixer means having a first input adapted to operatively receive the transmitter output and a second input operatively connected to receive the output of said variable filter means for combining the signals at said first and second inputs to produce an IF replica of said transmitter output; phase detector meanS having first and second inputs for producing an output indicative of the phase difference between the signals at said first and second phase detector means inputs, said first input being connected to receive the mixer means output; voltage controlled oscillator means for producing an output frequency whose phase is variable by means of a control signal input, said phase detector means second input being connected to receive the output of said voltage-controlled oscillator; amplifying means connected to receive said phase detector means output; filter means connected to receive the output of said amplifying means for providing said control signal input to said voltage-controlled oscillator; a plurality of band-pass filters connected respectively to receive said phase detector means output; and a plurality of threshold detector means connected to receive the respective outputs of said band-pass filters for providing respective outputs indicating whether the levels within the filtered bands exceed respective thresholds, the output of said plurality of threshold detector means being adapted to be connected to display means for indicating the conditions of the respective threshold detector means outputs.
 9. Apparatus according to claim 8 wherein: said mixer means is a Shottky barrier diode.
 10. Apparatus for measuring the spectral content of an external signal having a nominal carrier frequency, comprising: fixed frequency oscillator means; comb generator means connected to receive the output of the said fixed frequency oscillator for providing harmonics thereof; variable filter means operatively connected to receive the output of said comb generator means for selecting predetermined ones of said harmonics; mixer means having a first input operatively connected to receive the output of said variable filter means and a second input adapted to receive said external signal for combining the signals at said first and second inputs to produce an IF output frequency; phase detector means having first and second inputs for producing an input indicative of the phase difference between the signals at said first and second inputs, said first input being connected to receive the output of said mixer means, said phase detector means output being adapted to be connected to display means for indicating the level of said phase detector means output in a plurality of discrete bandwidths; voltage-controlled oscillator means for producing an output frequency whose phase is variable by means of a control signal input, said phase detector means second input being connected to receive the output of said voltage-controlled oscillator means; amplifying means connected to receive said phase detector means output; and filter means connected to receive the output of said amplifying means for providing said control signal input to said voltage controlled oscillator means. 